Such blades already known from U.S. Pat. Nos. 4,626,172 and 4,892,462, are of the type comprising:
a composite rigid shell, with aerodynamic profile, elongated longitudinally along the span of the blade, one longitudinal end of which, intended to be turned towards the hub of the rotor, has a blade root, the said shell including at least one layer of reinforcing fibres agglomerated by a matrix made of a synthetic rigidifying resin,
at least one spar, at least one part of which is housed substantially longitudinally in the shell, including at least one elongate composite bar of continuous and unidirectional reinforcing fibres agglomerated by a matrix made of a synthetic rigidifying resin, and
at least one filling body arranged in the shell between the latter and at least one spar.
Furthermore, in an example of a blade described in U.S. Pat. No. 4,892,462, each filling body is also a composite body, with reinforcing fibres aglomerated by a matrix made of a synthetic rigidifying resin.
In order to be individually dismountable and with variable pitch, each blade of the aforementioned patents includes a rooting part which is deformable in torsion about an axis substantially parallel to the longitudinal axis of the blade, and by which the latter is attached to the hub of the rotor. This rooting part includes at least one elongate composite torsion bar which longitudinally extends, outside the shell, at least one composite bar of at least one spar by passing through the blade root, which is tubular, the end of the torsion bar of the rooting part which is situated on the side opposite the shell being shaped into a loop surrounding a spool which is fixed removably to the hub by a bolt. In addition, the tubular blade root includes a metallic or composite cuff, the axial ends of which are each surrounded by one of two coaxial flanges allowing clamping and mounting in rotation of the blade root in two coaxial openings made in two walls of the rotor hub, the cuff also having, between these two flanges, a pitch control lever, projecting radially outwards and intended to be articulated to a device for collective control of the pitch of the blades. Inside the cuff, the blade root consists partly of extensions of the layers of fibres agglomerated by resin, constituting the rigid shell of the blade, optionally by extensions of layers of reinforcing fibres which are agglomerated by a synthetic resin, reinforcing the shell in its part close to the blade root, and partly by filling elements and/or layers of fibre fabric or plies of fibres which are agglomerated with synthetic resin in order to reinforce the blade root.
In the two aforementioned patents, all the composite elements, in particular the shell, the spar or spars, the rooting part, at least partially the tubular blade root and, optionally, the filling body or bodies, are made in composite materials based on polymerizable organic matrices consisting of thermosetting synthetic resins, for example epoxy resins. The reinforcing fibres of these composite materials are, for each blade, of different species, and in general of glass, carbon, or alternatively aramid fibres, and these blades may furthermore include certain elements consisting of synthetic, but not composite, materials, such as polyurethane foams.
This causes problems. It may be necessary to use several thermosetting resins on the same blade, each adapted to particular reinforcing fibres, and possibly non-composite synthetic elements. However, these various thermosetting resins must be mutually compatible. Above all, the thermosetting matrices, under the effect of the temperature and/or time, undergo a chemical conversion called polymerization. This reaction creates a modification in the molecular lattice, which becomes three-dimensional, unmeltable and irreversible. During this thermal cycle, these thermosetting matrices pass successively through three steps: liquid, gel, solid. The pieces based on thermosetting matrices adopt their final shape during the rise in temperature, just before gelling of the matrix. For example, for a matrix of the class of polymerizations termed 180.degree. C. polymerizations, this state is obtained at approximately 160.degree. C. Beyond this point, the matrix becomes solid, and the piece has adopted its final shape. This state is irreversible.
This irreversible and unmeltable nature of the state resulting from polymerization is the cause of a number of industrial difficulties of these blades: the reject rate is high, because manufacturing methods involving such polymerization are difficult to employ and sometimes time consuming, and lead to blades being obtained which do not always have the requisite dimensional characteristics. Because of the irreversibility of the polymerization, it is impossible to recycle the rejects, and repairs, for returning components to their standard form, are time-consuming, expensive and difficult, if they are possible at all. The chemical polymerization reaction can lead to the release of toxic gases, and the reactive nature of the products involved raises problems regarding shelf-life and storage of these products. Finally, it is known that composite materials with thermosetting matrices have poor fatigue strength, shock, impact and temperature resistance, and are sensitive to moisture-induced ageing.